var stream=require('stream');
var MyReadable=stream.Readable;
var rs=new MyReadable();
var str='a'.charCodeAt(0);

rs._read=function(){
    if(str<='z'.charCodeAt(0)){
        rs.push(String.fromCharCode(str++))
    }
    else{
        rs.push(null)
    }
}

rs.pipe(process.stdout)